1. Field of the Invention
The actual invention is related to the field of the development of pozzolanic materials for construction, specifically to microsilicas that exhibit high pozzolanic indexes and methods for their obtaining.
2. Description of the Related Art
The Portland cement is one of the most important materials in the construction industry due to its multiple applications and its advisable physical and chemical characteristics that present. Nevertheless, the costs associated to their obtaining as well as the high amounts that they must be produced to cover the necessities of cement for construction, have been important factors for the generation of new materials that allow to replace part of the cement used for the manufacture of concrete or products derived from concrete, without causing a decrement in their mechanical properties and resistance.
In this sense the pozzolanic materials have taken great relevance due to their capacity to interact with the cement components and to improve its properties.
The pozzolans are siliceous or silico-aluminous materials that in themselves have little or no cementitious value but, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to generate compounds possessing cementitious properties.
The application of the pozzolans in concrete allows to increase its durability in function of the increase in its diverse properties, such as impermeability, resistance to sulphate attack, handling, mechanical resistance in advanced ages and reduction in the alkali aggregate reactivity among others; these generates minor cement consumptions and the obtaining of construction materials with better mechanical properties and durability. With the application of such pozzolanic materials the consumption of energy for the manufacture of concrete and cement with improved properties is diminished.
The synthetic pozzolans, generated like byproducts from diverse industrial processes, have quickly become the primary source of artificial pozzolans used at the moment. The electrical power stations using rice husks or coal as fuel, and metallurgical furnaces producing iron, silicon and ferrosilicon alloys are the main source of artificial pozzolans like fly ashes, rice husk ash, blast furnace slag, silica fume, etc.
The silica fume, well-known like volatilized silica or condensed silica fume, is manufactured by electric arc furnaces as a byproduct of the production of metallic silicon or ferrosilicon alloys. In the transformation of quartz to silicon at temperatures of around 2,000° C., the gaseous SiO oxidizes to SiO2 and condenses in the form of special fine particles consisting of amorphous silica. The silica fume is removed by filtration of salient gases in filter devices.
The pozzolanic activity of the silica fume, based on its chemical reactivity with the calcium hydroxide of the cement, occurs substantially by the non-crystalline character of the silica and by its great specific superficial area (10 to 30 m2/g), that it depends on its particle size (lower than 1 μm). Nevertheless, the silica fume has a relatively low bulk density, so that the costs for their shipment and storage are relatively high. On the other hand, the silica fume form a great amount of dust and is difficult to make it flow; also it cannot be transferred into storage silos by pneumatic lines, bucket elevator or screen conveyor as easily as the cement can be.
On the other hand, the natural pozzolans are crude or calcined natural materials that have pozzolanic characteristics. Some natural pozzolans include volcanic ashes, pumicites, opaline cherts and shales, tufts and some diatomaceous earths.
The characteristics of natural pozzolans vary considerably, depending on their origin. This is caused by variable proportions of the active materials and their mineralogical and physical characteristics. Most of the natural pozzolans contain important amounts of silica, alumina, iron oxide and alkaline agents, which also react with calcium hydroxide and alkalis (sodium and potassium) to form more complex compounds.
The molecular structure as well as the amount of silica present in pozzolans is also very important. Generally, the amorphous silica reacts more quickly with calcium hydroxide and alkalis than does silica in the crystalline form (quartz, for example).
When a mixture of Portland cement and a pozzolan reacts, the reaction progresses like an acid-base reaction of lime and alkalis with oxides (SiO2+Al2O3+Fe2O3) of the pozzolan. This generates a gradual decrease in the amount of free calcium hydroxide by the formation of calcium silicates that will add force to the cement, and an increase in the CSH formation and calcium aluminate silicates that are similar to the products of hydratation of Portland cement. Has been found that the partial replacement of Portland cement by pozzolan works to increase the resistance of the concrete to sulphate and seawater attack, which is in part attributable to the removal of free calcium hydroxide formed in the hydratation of Portland cement by its combination with the pozzolan. The final result will be that the concrete mixture will contain less calcium hydroxide and more CSH and other products of low porosity.
The shape, fineness, particle size distribution, density, and composition of natural pozzolan particles, influence in the characteristics of freshly mixed and hardened concrete, and the strength development of hardened concrete.
There are several advantages when combining pozzolans with concrete. Concrete that contains a pozzolan typically has lower permeability; also the pozzolans have been used in low cement content concrete to reduce the temperature increase of the concrete, in comparison with concrete mixtures that contain Portland cement like the only cementitious material. The slower index of heat development with pozzolans allows more economic removal of heat in comparison with concrete that do not contain them.
On the other hand, almost any pozzolan when it is used in sufficient quantity is capable of preventing the excessive expansion resulting from the alkali-silica reaction. This reaction implies the interaction of hydroxyl ions associated with alkalis in Portland cement with certain siliceous components of the aggregates in concrete. The reaction products can cause excessive expansion, cracking, and the general deterioration of the concrete. It has been observed that the natural pozzolans are generally more effective than fly ashes to controlling the alkali-silica reaction.
The use of natural pozzolans with Portland cement for the concrete obtaining generally increases its resistance to aggressive attack by seawater, to sulphate solutions from soil, and to natural acid waters. The relative improvement is greater for concrete with low cement content.
The addition to the cement of a low quantity of pozzolanic siliceous material finely ground, generates insoluble salts that add cementitious value to the mixture; nevertheless, the calcination of the siliceous material is very important, since no reaction will happen between this material and the free lime unless the product has been treated later at high temperature, that is to say, treated under hydrotermic conditions.
There are multiple modifications to the pozzolanic materials, nevertheless, the particle size is one of the most important; by this way those materials with very reduced particle sizes are distinguished, which are called microsilicas. In this group are distinguished the synthetic silicas, the silica fume and natural silicas being these the most common in the market.
These products characterize by a high content of silica and particle sizes from 10 to 100 times smaller than the cement, which makes its application successful to make the concrete mixtures denser.
The silica fume is recognized like the main ingredient for high performance concrete, nevertheless, presents some disadvantages, such as:
a) The actual production is limited by smelting of silica,
b) The price by ton is high, and
c) It requires be used along with reducing water additives of high spectrum due to its lower particle size.
Unlike the silica fume, the suitably processed natural microsilicas, compete in quality with the silica fume at lower costs.
Like the pozzolanic materials, the microsilica allows to improve the cement characteristics, contributing to the improvement of a greater abrasion resistance, greater resistance to the chemical attack and a very low diffusion of chlorine ions. This allows that the resulting cements can properly be used in adverse environments, such as soils with high humidity or high sulphate contents, or marine environments.
Until now, multiple options have been generated with respect to siliceous materials, with natural or synthetic origin that allow increasing the cement durability. In this sense have been obtained silica mixtures with an ample distribution of different particle sizes1,2, hydrophobic silicas obtained from silicone oil precipitation3, silica mixtures with CaCO34, colloidal silicas5,6, silicas obtained like reaction products between bauxite and acids7, silica fume humidified dusts to improve their fluidity8, resulting silicas from the magnetic metal separation from rocky wastes9, synthetic microsilicas with high reflectivity10, stable watery dispersions of microsilica mixed with metal oxides11, microsilicas with bulk controlled densities12, microsilica dispersions that do not present thixotropic effect13, spherical silicas with specific diameters14, microsilicas mixed with chelating agents15 or treated with acids16, as well as silicas with tertiary structures from geothermal water17.
However, the mentioned siliceous materials previously are obtained from processes that involve the addition of multiple substances that can provide negative or undesirable effects when making contact with the cement. Also, the processes for their obtaining tend to be complex and to use great amounts of energy and infrastructure, which can increase considerably their costs.
Due to previous, is necessary to count with pozzolanic materials with advisable characteristics that allow to continue improving the mechanical and chemical properties of the concrete or cement, and that allow to be obtained by simplified processes from natural sources.